A Ti—Mo alloy having a beta phase with a body-centered cubic crystal structure as a main phase has advantageous features such that the corrosion resistance is excellent, shape memory properties are exhibited, and the Young's modulus is low, and an alloy having a Ti-15 mass % Mo composition has been mainly used. As examples of uses of the Ti—Mo alloy, there can be mentioned the use as a medical wire having shape memory properties as shown in PTL 1 and the use as a medical implant material as shown in PTL 2.
The Ti—Mo alloy is maintained at a high temperature such that the alloy is in the state in which a beta phase is solely present in the alloy (single beta-phase state), and then cooled to room temperature at such a high rate that no second phase (alpha phase) is precipitated so that the alloy is maintained in the single beta-phase state to room temperature, and the resultant alloy in this state exhibits especially high corrosion resistance.
However, the Ti—Mo alloy in the single beta-phase state does not exhibit a high value of yield stress at room temperature, for example, a Ti-15 mass % Mo alloy exhibits a yield stress at room temperature as low as about 400 MPa.
When the Ti—Mo alloy is subjected to heat treatment to precipitate an alpha phase with a hexagonal close-packed crystal structure, the yield stress is drastically increased to about 700 MPa as shown in NPL 1. However, the corrosion resistance of the resultant alloy is degraded, causing a problem in the resistance to crevice corrosion.
As a method for improving the Ti—Mo alloy in yield stress at room temperature while maintaining high corrosion resistance, a method has been known in which the Ti—Mo alloy material in the single beta-phase state is maintained at a temperature at which an omega phase of a trigonal crystal structure is precipitated, thus precipitating an omega phase (aged omega phase).
The omega phase (aged omega phase) precipitated by this method is so hard that the Ti—Mo alloy is remarkably improved in the yield stress at room temperature. However, the aged omega phase is a very brittle phase. Therefore there is a problem in that the precipitation of the aged omega phase in the alloy drastically reduces the ductility at room temperature.
There has not been a method for improving both the yield stress and ductility at room temperature with respect to the alloy in which an omega phase is precipitated. For this reason, in the conventional process for producing a Ti—Mo alloy, as shown in PTLs 1 and 2, the treatment temperature conditions and the composition of the alloy have been determined so that no aged omega phase is precipitated.
On the other hand, NPLs 2 and 3 have reported an example in which a structure in a swirly form is caused in a certain type of titanium-based alloy, i.e., a titanium-based intermetallic alloy to improve the ductility at room temperature.
For example, NPL 2 has reported that when a Ti—Al—Nb—Zr—Mo intermetallic-based alloy is subjected to hot extrusion, segregation in a swirly form is caused in the material, and the segregation causes a change in the order parameter of the arrangement of alloy elements, so that a hard portion and a soft portion are formed in the material, improving the ductility at room temperature.
The method for improving the ductility at room temperature described in NPL 2 utilizes a change in the order parameter of the arrangement of elements and can be applied to an intermetallic based alloy having ordered arrangement of alloy elements, but cannot be applied to an alloy having disordered arrangement of the alloy elements, such as a Ti—Mo alloy.
Further, NPL 3 has reported that when a Ti—Al—Nb—Zr—Mo intermetallic based alloy is subjected to hot caliber rolling and heat treatment in the temperature range where the second phase coexists, segregation of Nb and Mo elements in a swirly form is caused in the material, and further a specific microstructure in a swirly form such that second phase particles are precipitated in the areas in which Nb and Mo are dilute is obtained, and that the presence of the second phase particles increases the resistance to crack propagation during the deformation before breakage, improving the total elongation at room temperature.
However, the method of NPL 3 for improving the total elongation at room temperature has a problem in that the precipitation of second phase particles markedly reduces the corrosion resistance, and the like, and therefore cannot be applied to a Ti—Mo alloy.